DOMINANCE RELATIONSHIPS IN CAPTIVE MALE BARE-TAILED WOOLLY OPOSSUM (CALUROMYS PHIIANDER, MARSUPIALIA: DIDELPHIDAE) M.-L. GUILLEMIN*, M. ATRAMENTOWICZ* & P. CHARLES-DOMINIQUE* RÉSUMÉ Au cours de ce travail nous avons voulu tester en captivité l'importance du poids corporel dans l'établissement de relations de dominance chez les mâles Caluromysphilander, chez qui des compétitions inter-mâles ont été étudiées. Les comportements et l'évolution de différents paramètres physiologiques ont été observés durant 18 expérimentations effectuées respectivement sur 6 groupes de deux mâles et sur 12 groupes de deux mâles et une femelle. Des relations de dominance-subordination se mettent en place même en l'absence de femelle, mais la compétition est plus forte dans les groupes comprenant une femelle. Dans ces conditions expérimentales, le rang social est basé principalement sur le poids et l'âge. Lorsque la relation de dominance est mise en place, le rang social des mâles est bien défini et il reste stable jusqu'à la fin de l'expérimentation. Ces relations de dominance stables pourraient profiter aux dominants et aux dominés en minimisant les risques de blessures sérieuses. Les mâles montrent des signes typiques caractérisant un stress social : une baisse du poids et de l'hématocrite, les dominés étant plus stressés que les dominants. Chez les mâles dominants, la baisse de l'hématocrite est plus faible que chez les dominés, et la concentration de testostérone dans le sang diminue plus que chez les dominés. Au niveau comportemental, les dominants effectuent la plupart des interactions agonistiques << offensives » et plus d'investigations olfactives de leur environnement (flairage-léchage) que les dominés. De plus, leurs interactions avec les femelles sont plus nombreuses et moins agressives que celles des dominés. SUMMARY In this study, we test the hypothesis thal body size is a key factor in determining dominance relationships during agonistic encounters between captive males Caluromys philander, and male-male competition was experimentally tested. Physiological and behav­ ioural parameters were investigated during 18 experimental trials including two males alone (6), and two males with a female (12). A dominance-submission relationship exists even with no female, although competition is stronger when a female is present. Dominance is based on age and body mass in C. philander under experimental conditions. When dominance was settled, dominance rank was clear eut and consistent during ali the experimentation. Stable dominance-subordination relationships may benefit both dominants and subordinates by minimizing the incidence of serious wounds. Males show typical signs of social stress: bath body weight and hematocrit rate decrease, but subordinates are more stressed than dominants. * CNRS, UMR 8571, Muséum National d'Histoire Naturelle, Laboratoire d'Écologie Générale, 4, avenue du Petit Château, 91800 Brunoy, France. Rev. Écol. (Terre Vie), vol. 55, 2000. 337 Dominant males exhibit a lower decrease in hematocrit rate and a higher decrease in testosterone concentration in blood. They display most of the "aggressive" behaviours, scan more actively the experimental cages and engage in fewer aggressive interactions with females in comparison with subordinate males. INTRODUCTION Competition among males for access to females is often characterized by combats and aggressive interactions. These aggressive interactions are expected to play a major role in determining reproductive success of males (Krebs & Davies, 1997). But, under certain ecological conditions, mate searching by males could supersede combat as the primary mode of competition. This "scramble competi­ tion" polygyny has been first demonstrated in insects (Alcock, 1980) and seems important in sorne species of asocial mammals (Schwagmeyer, 1988). Even if combat and mate searching seem to be two contradictory modes of competition, in sorne species these two competition modes could coexist. For example, in sorne species searching is a prerequisite for acquiring mating opportunities. However, if numerous males accumulate around the same female before the onset of the œstrus, aggressive interactions can determine final suécess in mating (review in Schwagmeyer, 1988). Didelphids in general retain numerous ancestral morphological and physi­ ological traits (Charles-Dominique, 1983; Atramentowicz, 1986; Holmes, 1987). For these reasons, many authors have proposed that their social behaviour is more or Jess identical to that of their ancestors (Holmes,l 987). Social interactions have been well investigated in only one didelphid species: Didelphis virginiana (Holmes, 1987; Ryser, 1990). In this species Holmes (1987) and Ryser (1990) determined that captive and wild animais perform a stable social hierarchy, with male social status associated with body mass and testosterone concentration in plasma. Various features are associated with dominance in D. virginiana, including access to resources (food and mate) and scent marking (Holmes, 1987). In nature, females D. virginiana are followed by numerous males (up to five males; Ryser, 1990) from several days to several hours before the œstrus. This precopulatory accumulation of males around a female have also been recorded in the wild for another didelphid species: Philander opossum (Charles-Dominique, pers. obs.). The Bare-tailed Woolly Opossum (Caluromys philander) is a polygynous species. This noctumal, arboreal and solitary species is widely distributed in the northem and eastern regions of South America (Atramentowicz, 1988). Animais do not defend a territory, and male and female home range largely overlaps (Charles-Dominique, 1983; Julien-Laferrière, 1995). Individuals communicate through olfactory and auditory pathways, as in most noctumal species (Russell, 1984). In the Bare-tailed Wo olly Opossum social bonds are limited and hierarchy is not obvious. As in the other didelphid species, interactions between conspecifics are few, often aggressive, except during mating or mother/offspring relationships. Nevertheless, Caluromys occupies a special place among didelphid species, with a higher encephalisation quotient, related to arboreality (Eisenberg & Wilson, 1981), and extended mother-offspring relationships (Atramentowicz, 1986). Evi­ dence of inter-male competition to gain access to œstrous females has not been experimentally demonstrated, although it has been observed in the field (Charles­ Dominique, 1983). 338 In this study, we test the hypothesis that body size is a key factor in deterrnining dominance relationships during agonistic encounters between captive males C. philander. Experimental trials were performed in testing cages using paired males including, or not, a female. Morphological and hormonal correlates of dominance were assessed and we postulated that subordinate males would exhibit lower levels of testosterone and poorer physical conditions at the end of the encounter compared to dominant males. MATERIAL AND METHODS ANIMALS In this study, 12 females and 15 males Caluromys philander were used. AU of them were sexually mature (more than seven months) and born in captivity at the laboratory of Écologie Générale, M.N.H.N. (Brunoy, France). Owing to the little number of animais in our breeding colon y, males were used in more than one experiment. These males, used in more than one experiment, were individually housed for at least four weeks between two experimental trials. The diet consisted in fruit (apples, bananas and oranges) and a mixture of milk, baby cereal and protein-rich powder. Water was available ad libitum. Temperature (25 °C), hygrometry (50 %) and light period (12 h) remained constant throughout the year. In order to facilitate observations, animais were maintained in a reverse daylight. EXPERIMENTAL DESIGN Animais were housed in three adjacent cages (149 x 64 x 58 cm), each provided with a wooden nest-box and branches to allow locomotion activity for this arboreal species. Cages could be connected together, by opening doors. Doors were opened 48h each week during a four-week period. Therefore, the animais of an experimental trial could meet each other only during two consecutive days by week. Three experimental trials were monitored simultaneously in three separate rooms. Males were paired according to body mass differences. Both males were either about the same body mass (on average a 25 g difference, maximum = 50 g difference) or of different body mass (on average a 100 g difference, minimum = 80 g difference). In this species where adult body mass in males ranges from 300 to 500 g, these differences represented from 8 to 20 % of total body mass. Reference experiments were monitored with two males alone, and later on, another with two males and a female. Eighteen groups were tested: six groups with males alone, including four groups of different body mass and two groups of similar body mass; and 12 groups with two males and a female, including eight groups with males of different body mass and four groups with males of similar body mass. PHYSIOLOGICAL MEASUREMENTS Body mass was recorded weekly, two days before the beginning of the experiments. Blood sample of males was taken by puncture of the saphenous vein. - 339 - After centrifugation, hematocrit was measured and testosterone plasma concen­ tration was determined by radioimmunoassay. We used samples of 50 �-tl plasma and the procedure described by Perret & Atramentowicz (1989). The mean intra-assay coefficient of variation based on ali samples tested in